If you block a server using the "domain block" feature on Mastodon, this will cause you to lose any follows or followers you have on that server. The exact number of follows/followers you would lose are shown at the top of the warning when you do the block.
However, if you have done the block accidentally or if you have changed your mind about the block, here is a guide to how you can unblock the server and restore your follows and followers:
https://fedi.tips/help-ive-accidentally-blocked-a-server-on-mastodon-what-do-i-do-now
