Have you tried a spare key? If forcing the steering wheel hard left or right doesn't let the key turn then compare your key to a good spare and make sure it's not badly worn or damaged. If it can't lift the tumbler pins properly the cylinder will simply not turn to release the steering lock. Once the steering lock is released the key can be returned to the OFF position and the steering lock will remain released until the key is withdrawn. The key usually wears faster than the pins, especially cheap aftermarket keys (ones without the Volvo name embossed on the black plastic grip). Because the ignition pins do wear more than the other locks, a worn key will usually first be noticeable in the igntion while still opening the doors. Try lifting the key or pulling it out very slightly to see if that helps. Pulling it out sightly often works.

If you don't have a good spare then Volvo dealer service can actually cut (punch) a fresh new key using the key code. The odd locksmith may also have a suitable punch, but normally can only copy keys, wear and all. Depending on how worn the punch is, the new key may need heavy burnishing with a wire wheel to work properly in all locks. They can get the code from Volvo NA using the VIN if you don't still have the owner manual where it was normally noted inside a cover page. There should be no charge for getting the code, just for the key blank and a keying charge. Note that there are two types of key blanks, one that does the ignition, door locks, trunk/tailgate, glove box and wagon storage compartment, and what's called a "valet key" that only does the ignition and door locks. Lockmiths normally just carry the master blanks, but dealers carry both, so make sure you get the right one. The key cut is the same, just the blank keyways are different. The original owner received a set of valet keys in addition to the master keys and may or may not have passed them on. If you have one they are usually less worn and can be used for copying by local locksmiths. Valet keys were useful as emergency keys to hide on the car. If you're underneath the car poking around the bumpers or plastic wheel well liners or poking around the gas filler neck or front grill you might occasionally trip over a hidden key that has nice fresh unworn teeth.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now