On the assumption that this is an oil filled model that uses a refrigeration style compressor motor, here are a couple of things to consider;
If it has been sitting idle for a few years, its possible that there is there is a build up of corrosion on the valve plates, causing partial seizure. If your model has a removable lid (some are sealed), when you remove it you will see the motor shaft (vertical) with the piston attached & running horizontally, the head that the piston engages in should have a square block at it outer side held down by four bolts. Removing the four bolts gets you into the valve assembly, which is usually a set of flat machined plates & some U shaped parts, these should be quite easily removed & cleaned up if required
Another issue with these compressors is the wrong oil being used, which will gum up & cause problems. Again, if you can remove the lid & If the oil looks to be gummed up or solidified anywhere, you will need to tip the oil & clean the whole lot out with a suitable solvent, once you have done this & removed all the oil & solvent, let it sit for a while so that all the remaining solvent can evaporate & then refill with the correct oil. Sil-Air, Silentaire & all the rest of the silent compressor manufacturers sell high temperature synthetic oil specific to silent compressors & as far as I am aware the brand shouldn't matter much as they are all pretty much identical.
Silent compressor oil isn't that cheap, so you may want to refill it initially with regular compressor oil for a test run - but if it works OK, you will have to dump the oil, thoroughly clean & replace with the correct stuff.
Edit:- In this video, the head where the valve plates are located is highlighted at 1:02;
http://www.youtube.com/watch?v=nA-XbfxqgRo&feature=related