Your predicament is not uncommon amongst owners of older Macs. You probably know little about the CC's history before you became its owner, and what structural or electronic components were ready to break or fail, so all that you can do is go back to square one. Take off the case bucket. Reset the MLB (all power removed, including the battery) for 20min. Insert the battery and measure its voltage in circuit. If that is 3.3V or lower (regardless of lithium/alkaline), replace the battery. Hold in the CUDA switch for 40sec (it needs time to do its job, although the MLB was deliberately not powered for 20min) as your last act after reassembly before replacing the MLB's I/O door, or the case bucket, and then reattach the mains cable and switch on. Soak the CC on the mains again, trying at intervals to start from the keyboard.
If you cannot start up, you are firmly pointed in the direction of full-scale inspection/replacement/voltage-check, beginning with the mains cable and working through keyboard, SCSI power cable (for both 5V and 12V confirmation) and so on. If you can use known-good substitutions at any point, do so. If the RAM or VRAM has been augmented, try with both RAM cards removed (ie, return to base configuration), or with the single VRAM card out. If a substitution is not successful, put the original back before moving to the next test. Every potential failure point, even those that were new to Compact Mac construction—such as the flat-cable assembly—should be examined and cleaned. The FCA is the junction-point for MLB, drives and front pushbuttons, and therefore needs to be in place, intact and clean. If checking the voltages and components brings no joy, you may indeed be faced with replacement of components, boards or CRT.
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