Your lease probably has some clauses intended to shield the landlord from liability, and such clauses may have shifted the risk of this problem to you. If the flooding is caused by sources outside the landlord's control (land use patterns in the area, water escapement from other parcels, municipal drainage failure, and so forth), the landlord might not be liable. But a landlord's deliberate cover-up, knowing in advance that your business would be impaired by the problem, probably gives you a viable way around the lease clauses. Your question implies that corrective construction could fix the problem, and if so, you might leverage your potential claim into getting the landlord to undertake the construction. Evaluation of your situation requires more information than you can or should disclose in a summary on this site, including the paper trail and the course of parties' performance of your transaction. Also, it will be useful to know what your insurance coverage has to say about this. If this has caused, or is causing, or will cause significant financial damage to your business, it's worth spending something on confidential legal advice. The problems does not necessarily have to result in litigation, but from the summary disclosed in your question I predict that there are plenty of business lawyers, commercial landlord-tenant lawyers, and litigators in this area who would be interested in helping you.
This answer is intended as a courtesy only, and does not constitute an attorney-client relationship between the attorney and the questioner.