If market data is unavailable, you may see a red market data (DATA) icon on the upper right corner of TWS. This condition may be due to a firewall issue or to connectivity issue. You may use the keyboard shortcut CTRL+ALT+F to refresh the market data connections.
If that step does not solve the issue, please test the connectivity towards the primary TWS server/s using the steps on this page, considering that the primary server in a given region is responsible for distributing the market data coming from that specific region (e.g: the primary New Jersey server, distributes the market data of NYSE/NASDAQ, the primary Europe server, the market data for all European Exchanges, etc).
If the test returns positive results but you are still unable to receive market data, please contact IBKR Client Services to check if this is a local or general problem.