Angie,
You don't state what Zone you are in so I can olny speculate.
Lantana is hardy only to Zone 9, rarely Zone 8 (with lots of mulch), what you describe sounds more like a winter kill to me. I grow them as container plants in my Zone 5a gardens and carry them over in my basement light garden each year. In the Spring, after the last frost date they go back outside and I prune them back rather severly. New growth appears within a few days and the plants are putting on a show of bloom in about a month.
I prune the more robust growers after each bloom cycle and root the better pieces in moist potting mix. I keep the mix moist until new growth is evident and then I allow it to dry out in between waterings.
The only draw-back to wintering them indoors is that they are magnets for "White Flies".
I've never grown Plumbago, so can't help you there. If you're talking about False Indigo, which is a common name for it, I believe it will come back, you'll know in a few weeks. Of course, knowing what Zone you are in would help a whole lot!
Hemma