Not 100% but i believe this is steam related. Ive had this happen a few times over the years with CS:S and all Ive had to do is leave it for 20 mins or so and then comeback and its fine.
If that doesn't work, wait a little longer, its been a while since it happened to me, but it sounds like the same thing, so it should fix itself sooner or later.
Additional: (other possible reasons and solutions)
*Another copy is running in background: Ctr-Alt-Delete, bring up task manager and end HL2.exe then try again.
* clientregistry.blob error: Go into steam folder and delete clientregistry.blob. Restart steam and it will redownload clientregistry.blob, if this doesn't work you can try uninstalling and reinstalling steam. (http://forums.steampowered.com/forum.../t-565833.html
for more details)
*Antivirus software is blocking CS:S: Norton or McAfee used to cause problems but this hass supposedly been patched out, but could try deactivating just to test if they are causing problems.