Who do I contact about my SIM card suddenly not working?

I bought an O2 sim card a couple of weeks ago on the Smart M Bonus Plan. I activated it and everything was working perfectly for a week. Unfortunately, the past couple of days I can't make or receive phone calls, text messages or use data even though my phone says I have service and I have 500 mb data left. After a long time searching the site for someone to talk to, I found the chat function and someone who directed me to the correct department to deal with it, but then unfortunately my connection was lost. Another person on the chat gave me a number to call, but everything was automated and I couldn't get anywhere. Who can I contact to get this dealt with promptly? I'm expecting some important calls and this is very frustrating.