My computer no longer recognizes my password

Question

my imac does not seem to recognize my password.  now i cannot do anything. i never had install media.  what can i do?
Model Name:
iMac
  Model Identifier:
iMac7,1
  Processor Name:
Intel Core 2 Duo
  Processor Speed:
2 GHz
  Number Of Processors:
1
  Total Number Of Cores:
2
  L2 Cache:
4 MB
   Memory:
3 GB
  Bus Speed:
800 MHz
  Boot ROM Version:
IM71.007A.B03
  SMC Version (system):
1.20f4

Answer 1

Forgot Your Account Password
For Snow Leopard and earlier with installer DVD
     Mac OS X 10.6- If you forget your administrator password,
     OS X- Changing or resetting an account password (Snow Leopard and earlier).
For Snow Leopard and earlier without installer DVD
    How to reset your Mac OS X password without an installer disc | MacYourself
    Reset OS X Password Without an OS X CD — Tech News and Analysis
    How To Create A New Administrator Account - Hack Mac
Tutorial: Resolving Keychain Issues
If you can't access your keychain, or forget your password If you can't get into your keychain file because you've forgotten your password or the keychain file appears to be corrupt, there are a couple of options.
First, if you've forgotten your password, you can use the "Keychain First Aid" utility to make the keychain password the same as the login password. This can be accomplished via the following process:
  1. Open Keychain Access (located in Applications/Utilities)
  2. Go to the "Keychain Access" menu and select "Preferences"
  3. Click the "First Aid" tab
  4. Make sure the "Synchronize login keychain password" box is checked
  5. Close the Preferences window
  6. Go to the "Keychain Access" menu and select "Keychain First Aid"
  7. Enter your username and password
  8. Click the "Repair" button
The second option is to completely delete your keychain then recreate it. This routine is useful if your keychain appears to be corrupt or otherwise inaccessible. This can be accomplished as follows:
  1. Launch Keychain Access (located in Applications/Utilities)
  2. Click "Show Keychains" in the lower-left corner of the window.
  3. Select the problematic keychain from the left-hand pane.
  4. Navigate to the "File" menu and select "Delete Keychain '(name of keychain)'"
  5. Check all options for deletion and press "OK"
  6. Create a new keychain by going to the "File" menu, then "New" and selecting
      "New Keychain"
  7. You can now make this keychain your default if you desire by selecting it, then
      going to the "File" menu and selecting "Make '(name of keychain)' Default"
Login as root and perform repair In some cases, problems with keychains can only be resolved when logged in as the root user.
First, you want to enable the root user:
  1. OS X Mountain Lion: Enable and disable the root user
  2. OS X Lion: Enable and disable the root user
  3. Mac OS X 10.6: Enabling the root user
  4. Enabling and using the "root" user in Mac OS X
After enabling the root user, and logging in under this account, again open Keychain Access. First attempt repairs using Keychain First Aid, and failing that, delete then recreate the keychain as described above while logged in as root.
Persistently asked for stored passwords If you are persistently asked for passwords in various applications that you have specified should be remembered in a keychain, your "login" keychain may not be active for one reason or another.
Navigate to ~/Library/Keychains/ (this is the Library folder inside your user's home folder). Find the file named "login.keychain" and double-click it.
Failing that, select the "login" keychain within the Keychain Access application and make sure it is the default keychain by going to the "File" menu and selecting "Make 'Login' Default"
Turn off Keychain synchronization in applications having problems If specific applications are experiencing issues when accessing password-protected material, the Keychain may be to blame.
The above comes from an article published on MacFixit.com.