A commercial landlord facing default has several routes, and they are to a significant extent mutually exclusive.
Terminate the lease and recover possession. This ends the tenant’s obligation to pay future rent unless the lease preserves a claim for damages after termination — which well-drafted leases do, expressly.
Terminate possession without terminating the lease. Retake, relet for the tenant’s account, and hold the tenant for the difference. This requires clear notice of the landlord’s intent, because ambiguous conduct is construed as acceptance of surrender.
Do nothing and sue for rent as it comes due, available in states that impose no duty to mitigate on commercial landlords, and increasingly rare.
Acceleration clauses are enforced in many states subject to being tested as liquidated damages, and should discount to present value to survive.
Mitigation. Most states now require reasonable efforts to relet commercial premises. The landlord need not prefer the defaulted space over its own vacant inventory in every case, but must make genuine efforts and keep the record.
Self-help — changing locks without process — is permitted in some jurisdictions on strict conditions and is a serious liability elsewhere. It should never be done on assumption.
Security deposits, letters of credit and guaranties are applied under their own terms, and drawing on a letter of credit after termination requires the lease to say so.