Can not upload images / file etc

Hello all!

I run quite a few sites from the same server (20+) and all have developed the same problem over the past few days:

I can upload anything – images, documents, theme files etc – However, after ‘crunching’, the images appear as 0x0 in size and do not display. I’ve also just tried to install some new themes – and whilst they go through the process of uploading the ZIP file, I’m then given the error ‘Failed to write to disk’.

I have no idea what steps to take… any help much appreciated. This is quite a big problem for me, being that it affects every single site I run!