After six months of non-use KPN blocks your call credit. Top it up (within about three months) to unblock it.

After about nine months of non-use KPN cancels your call credit and prepaid phone number.

Put your prepaid phone number in a PRIVATE field of your forum/community profile, and signal here that you did this,

so a KPN forum moderator can tell you the phone number status.

If the phone number is important to you, try a top up as soon as possible.

Another possibility:

Sim card exchange required because of network updates.

See 

Use https://www.kpn.com/service/mobiel/simkaart-3/prepaid-sim-kaart.htm if free exchange no longer works.

Thanking for you message and reply, the above replacement SIM card option does not work, its screenshot is attached here, on the other hand the KPN’s Facebook told me that I can not retrieve my phone number any more, they closed or blocked my phone number without sending to me any warning, I had not received their message; I am so sad, meanwhile,  at the time I have faced many problems, in addition to, I used the above phone number and my call credit in connection with the DigiD over several organizations every month and I though that it was a kind of using of my call credit.

KPN’s Facebook told me, Following screenshot:
 

Get a new prepaid sim card with new phone number. Activate it by placing a short PAYED phone call. Put some extra call credit on it.

Login at mijn.digid.nl, change your phone number (a paper letter with activation code will be send to your BRP address, as registered by the municipality).

You may also look at the Digid App as login method.

Thank you very much for your help, I have already done it, but,  I was not able to change my phone number directly because my phone number was blocked and I could not receive any code to enter in my account and changing phone number, and so I had to ask for a new DigiD, I am waiting for the letter, but what I must do that in a future it does not happen again to me? they blocked my sim card  without warning, I had not received any warning, thanking you again.

I am sorry to read that your prepaid sim was blocked, ​@shahram-m. I understand this was due to not having used your phone number for more than 6 months, and then not topping up your credit within the following 3 months. We will always send you messages when the expiration date of your prepaid credit is nearing. In these texts we will also inform you about what you can do to keep your phone number. Unless you have blocked KPN, there is no reason why you should not receive these texts.

Regardless, we cannot retrieve your phone number. It is lost. The only solution is to order a new prepaid sim.

I also understand you would like to know how you can prevent this from happening in the future. You are to use your prepaid sim once every 6 months. To be save, I'd recommend using it once every 5 months. You use your prepaid sim when you send out a text message or place a phone call. Using a little data also helps.