We has had this problem in which all releases got stuck on TFS 2018
As there is a connectivity issue with SQL when release is completed , it may not update the status in DB in some cases if load is more, so the release is stuck in InProgress state and started consuming pipeline in SQL . Other releases will also not move ahead, as there is blockage in pipeline. Once we increase the pipeline count, the problematic release could move out as processing of releases started happening.
Once the problematic release is canceled by the system, we set the pipeline back to original count of 1, then you could see their releases progressing and not being stuck.
Solution:
You need to increase the count of pipeline to let say 25 after this create a new Release Pipeline and queue this pipeline this will push all those pipeline which got stuck .Once pipeline start queuing make the count back to one or original count.

Reference - https://blogs.msdn.microsoft.com/tfssetup/2017/11/14/understanding-build-and-release-pipelines-visual-studio-team-servicesteam-foundation-server/