askmyGP is the platform for patients & practices to communicate online, but the practice is in control of when the service is available for patients to do so.
If the practice are showing as 'offline' in askmyGP you would either need to contact them via other means, or login again the next working day when they should be back online again. Typically, first thing in the morning is the best time to submit a request, as some practices may choose to switch off access later in the day if they’re busy.
It might be that your practice are offline for clinical queries, but still permit requests relating to administrative queries, sick notes, or other non-urgent issues, the options which are made available to patients can differ from practice to practice.