When this happens to me, it's usually because there was a hiccup on the item server. To resolve this, I take small steps back to trigger a reconnect with the item server.
If joining a server and you see that you have stock weapons while others don't:
retry into the console. This instantly disconnects and reconnects you to the current server.
- If that doesn't work, disconnect and reconnect to a different server, such as an official Valve server. If you have custom weapons there and you go back to the original server and you don't, it means the server you're playing on can't communicate with the item server. Give it a few minutes, and it usually reconnects. You can also simply play on a different server if you're not terribly attached to a single one.
- If that doesn't work, restart TF2.
Generally you don't need to go past that, but if you really want to you can throw in the extra steps of restarting Steam and rebooting your computer.
If none of those work, this means Valve's item servers crapped a brick and you get to be patient until they bring them back online.