At first check the tracker status/message from your torrent client's tracker tab.

If it says "Not Working" then check the "Message" column. If it says certificate verify failed or skipping tracker announce(unreachable), then go to qBittorrent options(Alt+O) > Advanced > Disable the "Validate HTTPs tracker certificate" option > Apply and restart qBt.

If this does not fix your issue, then again go to qBittorrent options(Alt+O) > Connections > Tick the "Use different port on each startup" option.
If that option is not there, set the Port used for incoming connection to "0" (zero) and restart qBt.

Disable any 3rd party antivirus/firewall. Make sure to disable the firewall component completely. Also try disabling windows firewall.

If none of these work then use a VPN. If it works fine with the VPN then your ISP may have blocked our tracker. Contact with them to resolve the issue.