Sounds like you have not put the wheel back in properly and have somehow managed to over-tighten the freewheel/freehub, causing it not to spin freely.
You will have to take the wheel out and check that the axle nuts are done up properly and that the freewheel/freehub works. At a guess you are not using a quick-release lever, with nut-style axles you must tighten up each side gradually - 1/2 a turn each side once the wheel is in place or else you can tighten the axle (instead of putting the wheel in).
Do check for 'play' in the wheel once it is back in, if there is any then you will need to get it adjusted - wheel back out time... If you cannot work out what is going on with it, visit a friendly LBS during a quiet weekday to see if they can put it right for you whilst-you-wait.