The situation is that you have lost your password and you can't use the forgotten password feature to retrieve it. I will list a few different methods for restoring a password, I'd expect that at-least one of them will work for everyone!
Please read through the methods and determine which is most suitable for you.
Method 1: Using Forgotten Password & Database Queries
When? No mail server (working locally)
Head to the login screen and use the forgotten password feature. Enter your e-mail and click "Reset and Email Password" (I haven't forgotten that it won't work)
Pull up the database and find the "logs" table. This table contains a record of all e-mails sent (among other things). When you find your request, read the contents of the e-mail to get the reset link. EG: http://localhost/concrete5521/index.php/login/change_password/DgSniDsDTHDs/
Continue to reset your password normally.
Method 2: Manually modifying the password (126.96.36.199 and below)
Update. This will most likely only work on version below 188.8.131.52 and below. In version 5.6.3 the encryption was updated to use a more advanced encyption. If you are using 5.6.3 or over I would advise following method 3
When? Forgotten Password isn't functioning properly and version is below 5.6.3.
Open FTP and navigate to your site root. Open /config/site.php. Find the line that looks like ...
Copy down your password salt. In this situation, mine was gGSraUtFZ1mDtSirgKFmXKMu1gRWMARyUDSM4o8sBIlK9Djlb4kv2A5peUs1mPR6
Open http://www.adamek.biz/md5-generator.php and enter the password you want to reset to followed by a colon, followed by your password salt.
If i wanted to reset my password to "test", I would enter the following:
Remember that you need to replace it with your salt. After clicking "Calculate MD5", copy the text that appears in green.
Navigate to the Users table in the database, find the row of the User that you want to change the password for, modify the password column and enter your new md5 hash. Ensure theres no whitespace at the end.
Method 3: Using a script to change the super-admin password (ALL VERSIONS)
Have a read of the following thread: http://www.concrete5.org/community/forums/jobs1/password-reset-not-functioning/#378604
In there, jshannon posted a snippet of code that can be used to reset the super user's password which will also work!
I hope this helps - I've saved myself numerous times with the above methods when I've been working locally and forgotten passwords.