Go low tech before declaring it dead. With the battery in it and it turned on, hold it and give it a few whacks aginst your other hand. Sometimes oxidation builds up on contacts, and a jar is enough to get it going again.

The other thing is the battery might show a proper no load voltage, but the charging circuit puts close to a short circuit load on the battery at the start, and that might cause the voltage output of a weak battery to collapse to nothing. If it is lead acid gel cell technolgy and not regularly cared for I dont expect then to last much past 4 years.