You can try to use any other browser for the MetaMask login and see if the issue of MetaMask login persists or not. Also, another method is the uninstallation and the re-installation of the MetaMask Extension or the Mobile application, whatever you please to use. But if none of those common measures work, then the only option you are now left with is the use of customer care. Once you are on the support page of the MetaMask, you will be able to view the available communication methods. Select any one of those available methods and soon it will describe to you with further proceedings on how to solve your login issue.