I have lost the private key of my NEM wallet but i have password and username saved so i can get back my wallet. give me a helpful response if is possible solve my problem.
You must have not only password and username, you must have exported wallet file (.wlt) stored somewhere, and upload this file on the login page of XEM wallet - best is universal client with opt-in -https://github.com/nemgrouplimited/NanoWallet/releases/download/2.4.7-optin/build-2.4.7-optin.zip
Maybe there is a chance that the storage of your device is still ok and can be read out by some expert? That is probably the only way to get your private key.