I'd follow Bill's advice and start with the basics at the OBD diagnostic to see if the ECU is at least alive and able to function. A faulty ECU or ignition controller are well down the list of initial likelihoods for a no-start, but it's simple and easy to check the ECU. Although a good sign, anyone who opens up the ECU and says it looks healthy is only saying there's no obvious evidence of a burn spot at an overheated and likely fried component. There are so many other ways for electronics to fail.

FYI The odds are extremely good the ECU from a '93 940 B230FT would work in a '92 740 B230FT, both are LH 2.4 of similar vintage. Referring to Bill Garland's Volvo RWD ECU table (Google will find it for you), the '91-'93 LH 2.4 were seemingly compatible. If both engines have EGR then they may well both be the same p/n. If the '92 740 doesn't have EGR then an EGR ECU will likely still work, but can be expected to throw a Check Engine diagnostic and engine operation may be less than perfect. (Conversely, a non-EGR ECU in an EGR car typically won't throw a code.) I personally wouldn't bother doing an ECU swap unless you have stronger reasons to suspect the ECU as there's always the chance you'll accidentally weaken a contact or damage the tiny contact fingers.

At the diagnostic connector, check both controllers: port 2 for the LH 2.4 ECU and port 6 for the EZ-K ignition controller. If you plug the test lead in and push the button without the LED flashing a code then it's either a diagnostic socket wiring connection failure (not overly uncommon) or a possibly dead controller. If you get a faint red glow then there could be a poor ground or a failing component in the diagnostic section of the controller, while the engine may still run fine.

Next, read out and note any and all stored DTCs to indicate previously recorded faults. If the battery has been dead or the ECU disconnected there will be no codes, all will be 1-1-1. If any codes are present then read them twice to be sure then do a reset.

What you really want here is to run all the ECU diagnostic tests (DTM 1, DTM 2, and DTM 3) to check the controller is getting signals from the various engine sensors and it can control devices such as the idle valve, injectors, the fan (2 speed) and EGR solenoid (if so equipped). The procedures are all documented here in the FAQ
https://www.brickboard.com/FAQ/700-900/EngineOBDCodes.htm#FuelInjectionandIgnitionDiagnosticCodesi

After that, inspect all fuses, check for spark at each plug (better to do it in the dark to note a strong spark), make sure the fuel pump(s) briefly spin up when the key is turned ON (better to also check for fuel pressure at the fuel rail Schrader valve, if needed even using a tire gauge to check the approximate residual pressure is 30+ psi), attempt a restart and take your diagnosis from there.

--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now