Postpone
They may be unfit to work but still able to engage with a disciplinary hearing. It will depend on the reason for their absence. Postpone the hearing rather than immediately setting a new date. That will prevent the people who are conducting the meeting and taking the notes from making plans that they then have to change if the employee is still unwell. Remember employees can self-certify for the first seven days’ sickness absence.
Communicate
Illness doesn't prevent you from speaking to your employee. Tell them that you are postponing the hearing and speak to them about when they will be well enough to attend.
Inform
Tell your employee that if their absence continues, you will speak to them about how to continue and conclude the disciplinary process.
Explain
Point out that it is in their, and the company's, best interests to resolve the matter before memories fade and to reduce the stress that may be caused if it remains unresolved. Note that you may seek medical evidence on your employee's ability to attend, even remotely, a disciplinary hearing and fully engage with it.
Remind
Avoid having to chase your employee. Remind them to stay in touch in line with your procedures for managing absence. That should include telling you how long they expect to be away and to continue submitting statements of their fitness to work.
Tone
Remember to adopt the appropriate tone. Don't presume your employee is swinging the lead. An employee with a perfect attendance record may experience stress if they are worried about losing their job and perhaps their ability to work in your industry. Ask how they are. Wish them a speedy recovery.
Alternatives
If your employee is absent long-term with no imminent return, consider alternatives. If travelling is the issue, consider holding the meeting remotely through a video call. If they lack the equipment or software for a video call, consider holding a telephone conference meeting. If the employee is too unwell to take part personally, can they appoint a representative to act on their behalf? Or they may wish to submit a written statement addressing all the allegations and mitigation while you consider the matter in their absence.