- You do not stop the Sharepoint Central Admin service.
- You DO NOT stop the Sharepoint Central Admin service.
To get it back, you need to run the Sharepoint Products and Technologies Configuration Wizard. If you've already created a site, you'll be asked if you want to overwrite it, create a new site, or not to create a site. I chose not to create a site as I didn't want to screw up previous work. The wizard then ran through and at the end of it I had a working admin site again.
I tried some playing around with the STSADM command-line tool with the -createadminvs option, which looks like it ought to create a new admin website for you (I'm assuming the vs bit is short for Virtual Site) but I couldn't make this work and I couldn't find any discussion of this through Google to suggest what I was missing.