If you have utilized all of your committer slots, and a new user authors a VCS change, TeamCity Cloud will not start new builds with changes from that committer. Other builds will continue to start as usual.
If you have build credits available, they can be redeemed to add a new committer slot, or you purchase additional committer subscriptions through the JetBrains e-store.
We recommend you keep spare build credits in your TeamCity Cloud wallet for situations like these. Build credits purchased on-demand do not expire, and will allow you to redeem additional build resources at any time if needed.