If the agency program officer or grants officer is unavailable (or if the agency is closed) due to COVID-19, can grant personnel continue to work and charge salary to the project? 

In most cases, grantees should be able to continue work until their period of performance and funding is exhausted.  If prior approvals are required, the PI (with applicable institutional coordination and countersignature) should continue to submit their request to the funding agency using the normal mechanism for that agency (email or on-line system.)   In the event approval is not forthcoming within the time period for action allowed by the agency (typically 30 days), the PI will need to make a decision about whether to proceed with the action for which approval was requested, understanding that any costs associated with that change may need to be absorbed by the PI if approval is not subsequently granted.   For cooperative agreements and contracts that require agreement officer or contracting officer approval, the risk is likely to be higher.