2002 Volvo XC70 worked normally. Then wouldn't start

Do you have another key? Does that key also do the same thing?

Let me know what happens. Before you do any other repair you should try and get the central electronic module scanned for codes first to see if you have an immobolizer issue?

The tumbler has nothing to do with the starting problem. If the tumbler goes bad then you either can't turn the key all the way to the start position or the key will not come out. You either have an issue with the starter or the immobilizer. First, check the battery and make sure it is fully charged. You may want to take it out and have it tested. If that is O.K. Check for battery voltage at the battery cable on the starter after you have reinstalled the battery. If battery voltage is good then remove the solenoid cable on the starter. Get a volt meter or test light, preferably a volt meter to actually measure voltage. Have some one turn the key to the start position and see if there is battery voltage. If there is voltage when the key is turned to the run position see if you can manually turn engine over using a ratchet and socket. If engine turns over by hand then your starter motor has failed. If you have no voltage to the solenoid let me know. Maybe I could try and email you a wiring diagram. I would just need vehicle info, mainly engine size and if turbo or none turbo.

By the immobilizer I am assuming you mean the antenna ring you just did? But keep me posted as to what you find. If you need a wiring diagram then let me know.

Have you tried checking for power to the starter already?