The key was copied incompletely
The subscription link is a long string. If it was truncated while copying, the app sees no servers at all. Copy the key again with the button in your account rather than by selecting text.
The subscription expired
Check the expiry date in your account. After the paid period ends access is deactivated automatically, which looks like "server not responding" in the app.
Device limit reached
Each connected device takes a slot. When slots run out, a new device will not connect. Remove unused devices in your account.
The app did not refresh the server list
Clients cache the configuration. Tap "Update subscription" inside the app so the node list is re-read from the server.
Another VPN or proxy interferes
Two active VPN sessions on one device conflict. Turn off other VPN profiles and the system proxy, then connect again.
The network blocks the connection
Corporate and guest networks sometimes drop non-standard ports. Try another network (mobile data instead of Wi-Fi) — if it works there, the problem is the network, not the account.
Still stuck
Write to support from your account — the ticket reaches an operator together with your account details, no need to repeat them.