If you have "Dial From Home Screen" set to "No", all you need to do to lock the phone is press the "K" key.
There are other Home Screen shortcuts. A list of them can be found here:
Home Screen Shortcuts - BlackBerryFAQ
I thought that is the way you were locking it. How were you doing it before you got your BlackBerry fixed?
These are instructions for OS5 devices, but I think the older OSes will be similar. To set the "Dial From Home Screen":
1) Press the green phone key to bring up the phone application
2) Press the BlackBerry key (the key to the left of the trackball/pad)
3) Select "Options" from the menu
4) Select "General Options"
5) Change "Dial From Home Screen:" to "No"
6) Press the "Back" key (the key to the right of the trackball/pad)
7) Select "Save" from the popup menu
You also need to have a device password set to use the lock shortcut. Setting a password is located in the Options app. Let me know if you need specific steps on setting it.
--Ariel