First you mention $300, so can I assume that you are from the USA or perhaps made a purchase from the USA.
What I would suggest is for you to contact 'by telephone' PayPal direct and immediately, so that you can sort the problem out. The contact number should be available direct from the PayPal website or your account. Do not respond to any links in any email that you have received, for security and protection reasons.It could be a scam.
The thing that I like is the instructions: "You must add funds to your account and bring the balance to at least zero immediately". Perhaps I am over reacting, but that would make me slightly suspect, especially if it provides a link for a response from you.