Wrong microcontroller found

Hi, im using an Arduino mega 2560 and everything was working fine, but now that i try to upload the new version of my code the next error appears: Wrong microcontroller found, and in tools everything is ok, the right board and right port. At the end it appears this avrdude: Expected signature for ATMEGA2560 is 1E 98 01 Double check chip, or use -F to override this check. please tell me how to fix that, my project is due for next week thanks

http://arduino.cc/en/Guide/troubleshooting#toc23

I have had that problem with an Uno recently because a shield that I had plugged in was conflicting with the reset and serial pins.