You can check that the service as a whole is up using:
curl -I -k -X GET -H "Authorisation" -H "Cache-Control: no-cache" -H "Postman token" https://api.companieshouse.gov.uk/healthcheck
…which it is at the moment:
HTTP/1.1 200 OK
Date: Sun, 27 Feb 2022 16:42:20 GMT
Content-Length: 0
Connection: keep-alive
Server: CompaniesHouse
That doesn’t prove very much though.
Generally if there were any major issues here would be a good place to check.
I don’t think there are at the moment.
It’s most likely there’s an issue with your API key, or you’re trying to access from somewhere (IP / domain) that you haven’t registered with Companies House, or possibly you’re not using the “live” token (I’m not so sure about this part - we joined up before that functionality came in. Just search on the forum, there are questions answered about that.)