I have revelation 409 air forks, the 100 – 130mm u turn ones.

I have only just got them 2nd hand and they were fine. I turned the bike upside down before and since turning it back over the forks only travel about 20mm before feeling like they are locked out.
The poploc lever has been removed so there is just a little blue dial there now, dont know if that makes a difference though.
Could they need servicing or do i have an expensive problem?

Most likely a failed seal on the moco causing the oil to fill the lowers

Nice, i bought them hoping for reliability but instead get something that needs work immediately!
Is it a big job, do you know what is involved?

Fairly easy, its normally the seal on the motion control cart head.
plenty guides on internet- youtube