Check that the cable isn't shot and that the amp isn't broken. Plug in with a different cord, then try a different instrument to determine if it is the amp. Buy a new cord anyway, it's always good to have a backup around.
No advice if you don't have the know how other than take it to a reputable repair shop.
'73-4 Ampeg V4B, Ampeg PF350,PF115HE, PF115LF, 115 Traynor, 1962 Fender Jazz, 75 Gibson G3, '10 Squier VM Jazz, Squier CV 60's P Sonic, Squier CV 50's P blonde, Epiphone Thunderbird IV