Unfortunately: No, you can’t. You cannot reactivate it within six months. After that it will probably be possible again. No Flex subscriptions during these six months are available, but you can still travel with a balance, with or without subscription. ‘Dalvoordeel’ e.g. will give a reduction of 40 percent during off peak hours.

The suggestion of Momo below may apply to you.

thanks for this info….i still have the ns basic flex but i was thinking maybe i could switch to the Traject Vrij or Altijd Vrij because traveling with 40 percent discount everyday to work is not really a good idea. is there any possibilities to switch to this two options?( Traject Vrij and Altijd Vrij).

Both Traject Vrij and Altijd Vrij are available at the NS ticketmachine as a pre-paid ticket for 1 month or 3 months. See https://www.ns.nl/en/travel-information/traveling-with-the-ov-chipkaart/maandkaart-traject-vrij.html for more info.

These monthly tickets are not a NS Flex subscription. The price for a ticket for 1 month is the Flex monthly price + 20%, the 3 month ticket is the same price as the Flex monthly price. Also, with the pre-paid Traject Vrij ticket you don't get the 40% discount on other travels outside the traject during off-peak hours.

thanks so much for the heads-up please do you have any suggestion regarding what i can do? because i need to work every week and i will need an alternative..is there any other subscription i could do?

The TrajectVrij option of @Momo  is by far the best. AltijdVrij is only an option when you travel a lot. There are no other subscriptions besides the ones we mentioned. It is prepaid, but it will work.

No. But if you can tell us what trip you need to make for home-work, perhaps we can suggest-in some cases-an alternative that may work out a little cheaper.

IF you only use the train to travel home-work 5 weekdays AND if you know that you have fairly often a day off or whatever, then just PAYG=pay-as-you-go on prepaid balance is only marginally more as TRajectVrij paid advance for 1 month. But you probably can do the math all by yourself-check prices also via the NS site or via 9292.nl

If Traject Vrij or Altijd Vrij is the best option for you to go to work and you can't NS Flex for 6 months because you had a late payment, you can use the 3-month pre-paid card as explained above. After 2 periods of 3 months you should be able to get Flex again. 

Alternatively, have a look at Arriva Flow https://www.arriva.nl/consumers/season-tickets-and-tickets/arriva-flow.htm they have Dal Vrij and Altijd Vrij.

Traject Vrij can be bought at an NS machine, like @Momo said.

Also, it seems really strange to me if you still have Flex Basic, which usually costs more than traveling with a subscription (and subscription costs are always paid in advance!).

In a new topic today a NS moderator says that recently there was a change in this policy. Now when you have paid the invoice you can again apply for a new NS Flex subscription and do not have to wait 6 months. So that looks like good news for @sgt_vyper