For a computer as old as this one, you would have to ask someone with the necessary experience in reparing electronic devices. Do not automatically expect an authorised service centre to be able to handle it.
Would it be correct to assume that there is absolutely no reaction from the PowerBook when trying to power on? No sounds, and nothing at all on the screen? If so, try removing the battery. Resetting the Power Manager is probably not going to change anything in this case.
If still nothing, check the power adapter. Use a digital multimeter for this. The power adapter output voltage should be 7.5 V DC. The outer ring is minus and the centre is plus .If the power supply is OK, one would have to open the computer. There are four Torx screws in the lower piece and one Torx screw near the HDI-30 SCSI port. One suspect could be a soldered (surface-mounted) fuse on the main board (reference F1?) near the battery contacts. Other possible culprits could include leaking/damaged (surface-mounted) electrolytic capacitors.Jan
Someone with basic knowledge about electronics should be able to test the power adapter output to begin with. A standard digital multimeter is the only item needed. Measure whether the output is approx. 7.5 V DC. If faulty, one does not repair a power adapter (use another one instead).
It is not difficult to open the PowerBook 145. Search for disassembly guides for similar models at, for example, ifixit.com. Once open, have a close look at all circuit boards (use a magnifying glass). Apart from bulging/cracked/leaking electrolytic capacitors, look for discoloured or damaged components. Check that the resistance across a surface-mounted fuse (F1?) is zero (can sometimes be difficult to measure correctly because of interference from other components).