there may be alot of issues, making resold as you have, mmu chip die, glue chip die. Or any other chip.
First you start with psu, must be +5.00 volts and +12.00 volts
Then check somewhere if chips gets +5.00 volts on mainboard
Check if chips are getting power from PSU !
Then start to replacing/swaping chips with good ones, like mmu, cpu, glue...
Reseet chips not bad idea.
Unplug all cables from your atari, then start with reseet chips.
Tos, and all chips. Be carefull, and dont bend pins.
Use search in this forum- lots of ppl repair st by them selfs, sometimes then buy used chips (dont cost alot like max 20$) from other users to get his atari working.
If you guys expirience hardware problems.
1. Always remove chips if possible before soldering.
2. Resolder your hardware, check cables too.
3. If problem continue THEN must be faulty software.
I got 2 Atari ST
Main is stfm, with blitter, with 4 meg of ram, with 16mhz cpu + s-video and audio input to videobox, vdi out
My english may not be correct