Do the basics: check for spark and fuel.
SPARK:
When cranking does the tachometer needle move at all? If not it might indicate an ignition problem. Pull a plug (or preferably have a spare on hand) and with wire atatched, jumper or touch to ground the ground electrode of the plug and while cranking verify spark is present, should be strong and not yellow. If not,make sure all connections at the coil are in good shape, and verify you are getting power to the #15 (+) terminal of the coil with the ignition on, and that the hall sensor and it's wiring are good.
FUEL:
Jumper fuse #5 & #7, this will run the fuel pumps regardless of the igniton switch position.
For reference:
http://www.threefattigers.com/Protocore/Volvo/FuelPumpJump.jpg
You should hear both pumps running. If during a no start event jumpering the fuses gets you going, you likely need a new fuel pump relay.
If still not starting, being a turbo it's hard to get starting fluid sprayed into the intake, so try some carb cleaner through one of the vacuum ports while cranking. If it then tries to start and run, you have a fuel delivery problem which could be a number of things that are best sorted out with a Kjet fuel pressure test gauge. Barring the test gage, you can pull off the boot from the above the air flow sensor and with the fuel pump fuses jumpered, lift up on the air flow sensor plate, you should hear the injectors singing as fuel is passing through.
Make sure there is power to the frequency valve, sometimes these cars will not start if this part of the lambda-sond system is not working. With the ignition on there should be 12V+ present in one of the contacts of the plug for the freqency valve.