My computer wont turn all the way on?

it come to a black screen and then says load windows xp or xp safe mode i think and the it changes to another black screen stating last startup was not successful it gives me the option to either tart windows normally or last known good configuration it doesn't matter which one i choose windows xp starts to load and then it says monitor turning off and starts the process over by itself i installed no new software or hadware i stepped on the plug maybe and it turned off but the plug isntisn't problem i changed it and still does the same thing help