I have never had a problem with my Maryland state refund in the past. After not receiving my refund this year I remembered that I had not included my van I had sold for $1000 earlier in the year. If this is the problem, what can I expect to happen?

Possibly an audit. Why would you have had to declare the sale of a van?

Nothing will happen, because you likely paid a lot more than $1,000 for that van, which mean you had a capital LOSS on personal property which is NOT deductible.