How to get my PSP working again!?

Put a new UMD into my PSP and it said I had to upgrade to a 2.6 version?, I didnt want to do this so took the UMD out. When I put another UMD in it started to upgrade but had only got as far as upgrading 36%, I then switched my PSP off and now I cant get it to work, all I get is the green power light on for about 5 second and then it goes off. Please can someone give me any advice before I have to pay a repairer a fortune to fix it.