Can an employer unilaterally cancel a SEP retirement plan?

My employer is a small privately held company which is getting reqady to default on paying SEP to its employees. Generally this has been done each March, but now the company has become cashed strapped, and in March promised to pay SEP in September this year to the employees.
About 70 employees will be affected by this non-payment.
Can the company do this and what rights do the employees and laid off employees have in this situation. I believe that there was a contract as promises were made in writing on this subject.