Probably corrupted database by deleting records from node / node_revision
Oh dear, as you suspect that's not great. The outlook is probably quite bleak I'm afraid, a few vague possibilities:


If you know exactly what you deleted manually, restore it manually.
If you're able to restore a backup to another database, and cherry pick the records you deleted from that to be restored manually to the original DB, and depending on what damage those missing records have done in the meantime, that would possibly be an option.
If restoring the whole database from a backup is an option, do that.
If you know the IDs of the nodes you deleted, stick some dummy data with the appropriate IDs into the node/node_revision table, then call node_delete_multiple with the IDs so the system deletes them properly (clear the cache first).


The main problem with deleting straight out of the node table is that you leave a lot of orphaned data elsewhere - Drupal has no enforced referential integrity apart from some 'soft' checks in code for certain operations.

The most important thing to take away from this is never touch the database unless you're 100% confident what you're doing is safe. But I reckon you've probably learned that already :)