I opened Parallels and all that it shows is a black screen. I was using it and all of a sudden needed to reboot. When it did it showed the black screen. I rebooted my computer and it still doesn't work. I have no clue as to what to do. Please help me because I really need to use it. Thanks.
I'm going to assume some things since you don't give much information*: If this is a BootCamp install: Configuration Editor > Hard Disk 1 > Advanced > Clear (You'll have to reinstall Parallels Tools after, and probably re-activate Windows, if this is Windows). *Next time always state Parallels version, Guest OS type/version, BootCamp or not, Parallels Tools installed or not, and other relevant information. If Parallels is correctly installed, there should be a "Parallels Desktop for Mac User Guide.pdf" in there (/Applications/Parallels/).
Otherwise , backup VM as described in Parallels Desktop for Mac user guide from http://www.parallels.com/en/download/file/doc/Parallels_Desktop_for_Mac_User_Guide.pdf on page 243, and perform Windows repair as described in http://kb.parallels.com/en/5138