TLDR: You need to rollback to another version first and then helm upgrade again:
helm rollback <release> <revision> --namespace <namespace>
This can happen for a few reasons, but it ultimately occurs when there's an interruption during the upgrade/install process. Commonly, you SIGKILL (Ctrl C) while the deployment is ongoing.
You'll notice that if you helm ls --namespace <namespace> while it's stuck in STATUS: pending-upgrade state, you'll see the following without any other information:
NAME NAMESPACE REVISION UPDATED STATUS CHART APP VERSION
The best workaround currently is to rollback to another version, and then helm upgrade again:
helm rollback <release> <revision> --namespace <namespace>
revision is optional, but you should try to provide it.
more resources:
EDIT
If this is your first revision, use:
helm delete <release> -n <namespace>