What's probably going on is something referred to as a "blackout period" that generally occurs with corporate mergers or acquisitions.
When a blackout period occurs, the company is under obligation to notify you that your ability to make changes to your investments, take loans, or obtain distributions are suspended. I assume that's the notice you received that prompted your question.
During the blackout period, the company is waiting on the IRS to sign off on a new 401k plan that will cover the combined companies. Most companies give you an estimate of when the blackout period will end.
Here's a link to the IRS you may find helpful: Search "blackout": http://www.irs.gov/Retirement-Plans/IRC-401(k)-Plans-Operating-a-401(k)-Plan
If you aren't in the blackout period yet, make sure that your investments are appropriately diversified beforehand so that you'll be okay no matter how long the blackout period lasts. Hopefully it won't be long and the transition to a combined plan will go smoothly for you! Please reach out with any additional questions.