@mykie First thing I'd do is open it up and look for anything obviously burnt or crispy, any capacitor cans that look a lil top-bulgy. (Oh and if anything smells fishy or demonic, that could be failed capacitors which can literally go flash & pop)
If it's got a separate power supply, I'd use a multi-meter to check the output voltages on the plug meant to socket into the computer. Like, original C64 power supplies were notorious for failing in a way that sent way too much voltage