If you have just removed a load such as expansion or memory cards, or an internal hard disk, then the reduced load may be allowing the power supply voltage to rise too high, causing the over-voltage crowbar to activate.
If you have just added a load, the change in load may be overloading the power supply, or the new component may have a short circuit.
Take out all the cards, one by one, examine them, make sure the contacts are clean.
If your Lisa has a LisaLite card between the Lisa motherboard and the floppy drives, disconnect it as well - it may be backwards.
You should be able to remove both memory boards, the CPU board, and any drives, leaving only the power supply, CRT+video card, and the I/O board and turn the Lisa on. If it does turn on, you have a short somewhere on one of the removed items. Shut the Lisa off, and one by one insert each card/device; turn the Lisa on each time to isolate the bad card/device.
If you are unable to resolve this, and have a spare power supply, and that power supply works, check the large diodes and capacitors in the nonfunctional power supply.
The Lisa power supply has an output voltage setting which may prevent startup when in need of adjustment, see the appropriately named section for details.[LisaFAQ] > [Hardware] > [Power Supply and Power issues] (Comment on this answer)