as we read on the Eurostar website:
Industrial Action Belgium 22 April 2015
Please be aware that there is a national strike in Belgian on 22nd April. Early indications show that participation is expected to be widespread. The Belgian Railway Network is likely to be closed & an amended Eurostar timetable will be put in place (trains will terminate at and depart from Lille on the continent). As a precaution, sales for Eurostar tickets to & from Brussels are currently closed for sale for this date.
Further information will be provided next week.
Updated 20 March 16h00 GMT
Please be aware of travel inconveniences. Consider alternative means of transportation.