Did you happen to move the car without warming it up?

If so, you had 'lawnmower syndrome' (do a search, it is common) - you move the car to get the lawnmower out, then it won't start.

Volvo white (aluminum) 5 cylinder engines suffer from this.

Raw fuel on the rings or cold valve guides cause a low compression and a no start.

The customary solution is to crank and crank until it starts. It may stumble and stumble during this effort.

Often it cranks a little faster than normal.

If it gets towed to a dealer, they install new plugs and change the oil because it has fuel in it. (Ask me how I know that!)

Check your oil, if it is high - change it.

After my episode, I kept a new set of plugs in each of our 3 850s,

AND we had a family rule - never start an 850 unless you warm it up fully.

Sounds like it is running 'rich' and eventually fouling the spark plugs. Unless the old plugs were really worn out.

A rich condition is normally the result of one of the vacuum lines developing a leak. Has the PCV system ever been serviced? One way to find out if the system is not creating enough vacuum is to take an old latex glove, remove the oil filler cap, and cover the filler cap with the base of the glove. A rubberband may be needed. Then start the car. If the glove inflates, then you have a problem.