Work in order, cheapest first
Login failures have a small number of causes and it saves time to rule them out in order of effort. Start with what you typed, then the connection, then the app, then the account itself. Most people contact support at step one when the answer is at step two.
Credentials
Retype the mobile number with the correct country code and no spaces. Reveal the password before submitting. If a password manager filled the field, check it is not an older saved value. If you changed the password recently on another device, the old one will be cached in several places.
Connection and device
Switch between mobile data and Wi-Fi; a captive Wi-Fi portal will silently block the request. Disable a VPN if one is active, as some platforms reject connections from unexpected regions. Check the device date and time, since a clock that is badly wrong breaks secure connections.
App-side problems
Update to the current build. Clear the app cache — not the account data — and reopen. If it still fails, uninstall and reinstall the current package; the account is stored on the server and survives this. As a test, try signing in through a mobile browser: if that works, the problem is the installed app rather than the account.
OTP and verification codes
Codes arrive by SMS and can be delayed by the network. Wait a couple of minutes before requesting another, because rapid repeat requests often trigger a temporary cooldown. Check any spam or blocked-sender filter. If codes never arrive at all, the number on the account is probably not the number you are holding.
When it is the account
A repeated failure with credentials you are certain about, or an explicit message about restriction or suspension, points at a server-side decision. Only the operator's support can review that, and they will ask for the registered number, the user ID and the registration date. This website cannot unlock, restore or verify any account, and anyone who offers to do so for a fee is running a scam.
Where to go next
For account access see the login guide, for creating an account see the registration guide, for the mobile client see the app page, and for short answers see the FAQ. If the amount of time or money involved has started to concern you, please read the responsible gaming page.