Today I was looking for a particular place in the code file and found that there are no current line number displayed anywhere in the IDE. :S
The reason was that there was no status bar displayed at all.
Tools -> Options -> Environment -> General -> ShowStatusBar is the option to turn it back on. This has not happened ever since I've been using Visual Studio products. Wasn't even aware that such a thing is possible. :)
Edit: Thanks for the feedback, everyone! I've updated the settings path and the option to enable line numbers in the editor.
If, instead of enabling the status bar, you are looking on how to enable line numbers in the editor window, on the left-hand side, try the following option, as suggested in the comments below:
1. Tools > Options > Text Editor > All Languages > General
2. On the right, under Display section, put tick mark for Line numbers.