I have noticed that RF24.h and SPI.h have a habit of being there one day and not the next. I have always reloaded them from Sketch --> Include Library --> Manage Libraries. (I would like to know why they disappear! ) but -Shock! Horror! today, neither of them is available that way. Obviously, without them I cannot compile and run my project work. Hence the Emergency!
In the Arduino document Installing Additional Arduino Libraries I followed the steps for importing a .ZIP library, having downloaded RF24-master.zip from [u]https://codeload.github.com/TMRh20/RF24/zip/master[/u]. I opened it as directed but was then told to "Return to the Sketch> Import Library menu." My IDE (1.6.9) does not have a Sketch > Import Library menu but RF24 does show up in Sketch --> Include Library. Compiling gets me
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c: In function '__start_timer':
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:37:2: error: 'TCE0' undeclared (first use in this function)
 TCE0.CTRLA = TC_CLKSEL_DIV256_gc;
 ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:37:2: note: each undeclared identifier is reported only once for each function it appears in
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:37:15: error: 'TC_CLKSEL_DIV256_gc' undeclared (first use in this function)
 TCE0.CTRLA = TC_CLKSEL_DIV256_gc;
       ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:40:15: error: 'TC_WGMODE_NORMAL_gc' undeclared (first use in this function)
 TCE0.CTRLB = TC_WGMODE_NORMAL_gc;
       ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:47:18: error: 'TC_OVFINTLVL_HI_gc' undeclared (first use in this function)
 TCE0.INTCTRLA = TC_OVFINTLVL_HI_gc;
         ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:50:2: error: 'PMIC' undeclared (first use in this function)
 PMIC.CTRL = PMIC_LOLVLEN_bm | PMIC_MEDLVLEN_bm | PMIC_HILVLEN_bm;
 ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:50:14: error: 'PMIC_LOLVLEN_bm' undeclared (first use in this function)
 PMIC.CTRL = PMIC_LOLVLEN_bm | PMIC_MEDLVLEN_bm | PMIC_HILVLEN_bm;
       ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:50:32: error: 'PMIC_MEDLVLEN_bm' undeclared (first use in this function)
 PMIC.CTRL = PMIC_LOLVLEN_bm | PMIC_MEDLVLEN_bm | PMIC_HILVLEN_bm;
                ^
C:\Users\Rick\AppData\Local\Temp\build80a800aba6da0fc49c9b73fed8b532db.tmp\sketch\libraries\RF24-master\utility\ATXMegaD3\compatibility.c:50:51: error: 'PMIC_HILVLEN_bm' undeclared (first use in this function)
 PMIC.CTRL = PMIC_LOLVLEN_bm | PMIC_MEDLVLEN_bm | PMIC_HILVLEN_bm;
                         ^
Using library RF24-master at version 1.1.7 in folder: D:\AdDipMechEng\_Semester_2_2016\Select_Remy\Programming\Arduino\GlasshouseTraversingSystem\Trolley\libraries\RF24-master
Using library SPI at version 1.0 in folder: C:\Users\Rick\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.6.13\libraries\SPI
exit status 1
Error compiling for board Arduino/Genuino Uno.
SPI.h is a core library. Unless you've been going in and deleting files, it won't just disappear. Your compiler output says it is finding both RF24.h and SPI.h, but it is using some older versions that you must have pulled in from somewhere. It's throwing a bunch of errors about you using names that you haven't declared anywhere.
Why don't you show us the code that produced these errors and the header files you're using for SPI and RF24. It seems like if it really is an emergency, you'd want to give us information that would help solve the problem.
I used Agent Ransack to scour C:\ for files with a name containingSP24. Aside from a string of files cached by my browser (MS Edge) there is only RF24-1.1.7.zip. However, some strange things happened.
RF24 is now visible again via Sketch --> Include Library. I don't know where it went, but it's back!
I found an anomaly in my folder structure thus:
GlasshouseTraversingSystem
 - Console
  - libraries
   - RF24
    -examples
    - examples-linux
    - pyRF24
    - tests
    - utility
 - Trolley
  - libraries
   -RF24-master
    - examples
    - examples-linux
    - pyRF24
    - tests
    - utility
I shifted the two subfolders without the libraries thus:
GHTS
 - Console
 - Trolley
and, low and behold, my two sketches compiled, downloaded, and ran. :o
Those two libraries folders must have appeared somehow while I was trying to make up for the absence of RF24 from Sketch --> Include Library.
Sometimes I think I am being haunted, but there seems to be some foundation for that in fact. I don't know what cause the problem originally, but for the time being it seems to be resolved.
I'll get back to this thread it the issue rears its ugly head again.
) but -Shock! Horror! today, neither of them is available that way. Obviously, without them I cannot compile and run my project work. Hence the Emergency!