From time to time, the wrong series of events will conspire to keep MPLAB® X IDE from running. Anytime PCs are involved, there are several possible culprits, but some of the more common causes and solutions are discussed below.
You just upgraded to a new version
After installing a new version of MPLAB X IDE, one of the last dialogs instructs you to restart your computer. If you forgot this step, the IDE may not start, or if it does it will not function properly. Make sure you restart your machine after a new installation or upgrade.
The MPLAB X persistence files have become corrupted
MPLAB X IDE maintains a variety of settings such as the last open project(s) and other session data in a series of persistence files. In the world of PCs, any number of events can cause file corruption: crashes, driver errors, faulty programs/utilities, and anti-virus software among many others. MPLAB X can be reset to is default state by deleting its persistence files. The next time you run MPLAB X, it will create new persistence files and everything should work again.