Unless you found this article using a search engine, which means you’re probably having the same problem I did, you’re very unlikely to be interested in its solution, so you can stop reading now and get on with your life.
Certain SIM cards have to be unlocked by typing in a PIN every time you turn your phone on. For example, the Fonic SIM I’m using here in Germany. If you type the PIN in wrong three times, you get put into “PUK lock”; to get out of it, you have to type in the much longer “PUK code”, which is printed on the carrier the SIM card came in.
Only on a Nexus One (one running Android 2.2, anyhow) you can’t. There’s some toxic interaction with the unlock screen, and various settings reconfigurations couldn’t fix it, nor could the telephone company‘s tech support.
So here’s the answer: Take the SIM out of your Nexus One, find someone with another kind of phone (a primitive no-frills Nokia worked just fine) and have them cycle the power: You’ll be able to type in the PUK and your phone will come back to life.
“Wait,” you cry, “how on earth did you screw up your PIN three times in a row?” Clever of you to notice that I left that part out.