Understand the reasons behind the delay. Is it engineering complexity? Scope creep? Departure of a key team member? Platform switch? Data migration? Change in competitive landscape requiring change in features? This step would involve a detailed conversation with the engineering leads.
A meeting with the engineering leads, business owner, and yourself would be the next step to make sure everyone is on the same page. Brainstorm if there are any technical resources/solutions that could get the launch date back on track.
Determine who will be affected and what actions need to be taken. Customers, business owners, public relations, and other product teams are the most likely parties impacted. The worst case scenario is if the delayed product was actually sold to customers prior to launch. Unfortunately, this is done all the time in the technology world. In that case, pre-sold customers will need a refund or personal calls from your team to assure them corrective actions are being taken and the product will be delivered.
A new launch timeline has to be created with the changes in resources, priorities, and complexity.
Execs need to be brought up to speed. Customers should be briefed. Marketing needs to be told not to promote a product or feature that doesn't exist. Business owners need to adjust their yearly budget and goals. Sales needs to know what expectations to set with potential customers. PR needs to know not to publish that press release that's been prepped for weeks.