Spinning Cassette

There's a lot of sense in the above posts and, if you combine it with watching a couple of the YouTube videos on the subject, you will be able to flush out your freehub without stripping it down, re-oil it and bingo, you're back on the road.

Best of luck.

Freewheel has seized albeit intermittently. Replace from hubs are usually available. Water , dirt corrode latching pawls

Perhaps you have a freewheel if it's an older or cheaper modern bike, but in all likelihood, if its a modern bike with 8 gears or more then you have a cassette and a freehub.

Good case scenario:  Its just gummed up with all that badass winter cycling you have been doing.  If you have the tools to remove the cassette and the axle (cassette removal tools, cone wrenches, adjustable spanner and an allen key normally), its usually not difficult to remove, clean out with degreaser and GT85 or whatever, regrease and reassemble.  I'm sure there are awkward freehubs out there, but lets be optimistic.  What you are looking for is a small set of engagement teeth (pawls) that should spring out once cleaned, they may be caked with grit and mud, but once scrubbed up you're fit to go.

Worse case scenarios: The spring has gone on your freehub and no amount of cleaning will get it to work again.  It may be possible to replace the freehub and that's going to cost from around a minimum of £15, average £30, more exotic - lots of ££££.

Alternatively, the inner surface of the freehub has worn out - possible if its soft alloy, its the same prescription as for broken spring.

Worst case scenario:  Its both a broken hub and you can't source a replacement!  If this is the case, then you'll need a new hub and a wheelbuilder, or if it's a relatively inexpensive factory wheel, a replacement wheel.  A pair of perfectly servicable Shimano 501s costs £68 from Ribble. 

Depending on your wheel, replacing the hub may be the most cost effective option.  A Tiagra hub costs around £22, 105 £30ish, add £25 for a rebuild and you'll be back on the road for about £50.  A wheelbuilder will advise you on whether your rim is worth saving. 

If you don't have any tools, the cost will quickly mount up and make a local bike shop or wheelbuilder an attractive option.  If you do have the tools, there are plenty of folk on youtube who have put up "how to" guides.

Fingers crossed its just clogged up.

I think this it is the freewheel which has broke which is what happend to me. 

I needed to get a replacement from a LBS.  Can't be sure if they are repairable.