There's good news and bad news. The good news is that you might well have forgotten that the default password on the Samsung SGH-D500 phone is 00000000 (that's eight zeroes). The bad news is that if you've changed it to something else and that's what you've forgotten, you might well need to take the phone in to a service center to have them help you break it back open.
First, here's what Samsung says about working with passwords on the D500:
Security
Use this menu to protect the phone against unauthorized use by managing the several access codes of your phone and SIM card.
Note: If you enter an incorrect PIN/PIN 2 three times in succession, the SIM card is blocked. To unblock it you need to enter your PUK/PUK2 (Personal Unblocking Key). The codes are supplied by your network operator.
PIN check
The 4-to-8-digit PIN (personal identification number) protects your SIM card against unauthorised use. When this feature is enabled, the phone requires your PIN each time it is switched on.
Change PIN
Use this menu to change your PIN. The PIN Check feature must be enabled to use this feature.
Phone Lock
The Phone Lock feature locks the phone from unauthorised use.
When this feature is enabled, you must enter a 4- to 8-digit phone password each time the phone is switched on.
The password is preset to 00000000. To change the password, use the Change Password menu.
Change Password
Use this menu to change your phone password. The password is preset to 00000000.
Privacy
Use this menu to lock your media contents, including MMS messages, emails, images, videos, music files and sounds:
1. Press [OK/I] to check the item(s) to lock.
2. Press Save to lock the marked items.
3. Enter the phone password and press OK.
To access a locked item, you need to enter the phone password.
If that doesn't help you, as I said, you'll need to take your phone to the local service center and ask them for help. Just a tip: Bring some sort of proof of purchase or ownership so they can differentiate between you losing your password and someone else showing up with a stolen telephone
First, here's what Samsung says about working with passwords on the D500:
Security
Use this menu to protect the phone against unauthorized use by managing the several access codes of your phone and SIM card.
Note: If you enter an incorrect PIN/PIN 2 three times in succession, the SIM card is blocked. To unblock it you need to enter your PUK/PUK2 (Personal Unblocking Key). The codes are supplied by your network operator.
PIN check
The 4-to-8-digit PIN (personal identification number) protects your SIM card against unauthorised use. When this feature is enabled, the phone requires your PIN each time it is switched on.
Change PIN
Use this menu to change your PIN. The PIN Check feature must be enabled to use this feature.
Phone Lock
The Phone Lock feature locks the phone from unauthorised use.
When this feature is enabled, you must enter a 4- to 8-digit phone password each time the phone is switched on.
The password is preset to 00000000. To change the password, use the Change Password menu.
Change Password
Use this menu to change your phone password. The password is preset to 00000000.
Privacy
Use this menu to lock your media contents, including MMS messages, emails, images, videos, music files and sounds:
1. Press [OK/I] to check the item(s) to lock.
2. Press Save to lock the marked items.
3. Enter the phone password and press OK.
To access a locked item, you need to enter the phone password.
If that doesn't help you, as I said, you'll need to take your phone to the local service center and ask them for help. Just a tip: Bring some sort of proof of purchase or ownership so they can differentiate between you losing your password and someone else showing up with a stolen telephone