[Simon Lloyd]

I dont think you can, you need to look at the tmp folder on your server, that may be full. What you could do, if your forum is in a folder, is just create a simple html file, call it index.html, create a new folder at the same level as your forum, rename you forum folder and name the new folder as your forum, drop in the index file you just made (of course this will have an apology on it to your members while you sort things out) and then you can try accessing www.mysite.com/myrenamedfolder/admincp and see if you can get on but really you need to get your host working on this if you dont have a dedicated box!