We connect to the repository by ip address - a quick hack introduced by the guy before me, since we don't have a real server, just an old pc running apache, svn etc. We recently moved offices and it seems the "server" is using DHCP - it booted to a new IP address this morning. Logging into trac (also running on that server) is easy: Just change the bookmark in the browser.
But what do I do about my working copy? How can I tell that where to find the server?
Do an svn switch and use the --relocate option:
http://svnbook.red-bean.com/en/1.5/svn.ref.svn.c.switch.html
(or in TortoiseSVN, right-click -> TortoiseSVN -> Relocate...)
The relocate does not require repository access and is meant exactly for the case where the content is the exact same but the repository base is changed.
Davs answer points to the right chapter, but to be more precise, its actually the command
svn switch --relocate [old URL] [new URL]
which maps to the TortoiseSVN command TortoiseSVN -> Relocate
Dav's answer (using svn switch with --relocate) is the proper answer, but before you do that, I have one other suggestion: Use a DNS name. Since it sounds like you don't have a DNS server, add an entry to your hosts file pointing to the IP address. Then if the IP address changes again, all you need to do is have each person update the hosts file on there machine.
Consider setting up DNS for the subversion server - you can use an internal DNS server, a free DNS hosting service, whatever. Doesn't mean the server has to be publicly accessible across the internet, you can have the DNS lookups return a LAN IP. That'll make it a lot less painful the next time the server changes IP.
When that part has been done, relocate the repository location like described in other answers.