I suddenly started getting this error a couple days ago:
loading hardware from E:\Arduino\hardware: loading package arduino: loading platform release: loading E:\Arduino\hardware\arduino\avr\platform.txt: Error reading file: open E:\Arduino\hardware\arduino\avr\platform.txt: Access is denied.
Error compiling for board Arduino Uno.
I've confirmed that the user that the Arduino.exe process is running under is in one of the groups that has full control of the file. I've also tried doing a full uninstall and reinstall of the Arduino IDE as well as turning off my antivirus to see if that worked. So far, nothing has worked. I cannot verify or upload anything from the IDE at this point, which is a major problem for me as I need to be able to do so for school course work. Any help is greatly appreciated at this point.
Update: I still don't know why this issue was occurring (I'm thinking there may have been some residual files and settings left from the initial uninstall), but I've uninstalled the IDE and reinstalled it to a different drive (C: instead of E:) which has resolved the issue. It doesn't tell me why it was occurring to begin with, but maybe when I get some time and am not under time constraints for class work, I can circle back to it and try it again on the E: drive.
This sort of error could be caused by your antivirus software.
When you circle back to investigating the issue and are able to reproduce it again, try TEMPORARILY disabling your antivirus for a single compilation to see if the problem goes away, then turn the antivirus back on. If the problem doesn't occur with the antivirus off you will need to adjust the settings of your antivirus to whitelist the appropriate file, folder, or process so it doesn't interfere with compilation.
Please be cautious about working without an antivirus. This is only about temporarily disabling it for a quick test. If you don't feel comfortable doing that, fine. You can try going straight to configuring the antivirus to whitelist the Arduino software.