Red cluster status master not discovered exception

we have facing issues of red cluster status and out of 3, only 1 node is visible.
before that, we have t2.small instance and to resolve the red cluster status, we have upgrade instance type t2.medium
still, clusters are in red and upgrading instance type showing in-progress.

How I resolve it? and how I get back all 3 working nodes?

for now i got message master_not_discovered_exception

or sometimes I also get a message

cluster_block_exception

can anyone help here?

Which version of Elasticsearch are you using?

How are the nodes configured?

How did you end up in this state?