They may be waiting for some kind of medical report.
Did you (or any travelling companion) contact them as soon as possible after you had the attack, or did you wait until you returned home before letting them know? If the latter, it may be considered you have breached one of the terms of the insurance policy - look at the policy to see what it says. However, if you were travelling alone and the state of your health as a result of the attack was such that it was not practical for you to notify them sooner than you did, this should be accepted.
If you can't get a result, look at your policy to find details of the complaint procedure & follow that - ending up with the Ombudsman if necessary.
This will take time and - if you can't afford to pay the Visa bill - you will have interest etc. added to it. This should be part of your claim - i.e. the delay in settling caused you to have the interest etc. added.